The relation between extension and impenetrability is a major issue in the Descartes-More correspondence, which implies an analysis of the concept of extension. In the course of the attempt at characterizing it, the mereological structure partes extra partes is a crucial element. Both philosophers hold two opposed views of this mereological structure. I try to show that these two views can be traced back to scholastic discussions on quantity’s relation to extension. This background provides a vantage point, which enables to propose a new construal of the argumentative exchange around impenetrability, and which casts some light too on our understanding of two properties – spissitude and indiscerpibility – which More attributes to spiritual extension in his later writings.